Abstract

The removal of dissolved manganese in water treatment floc blanket clarifiers has been studied. The removal mechanisms may be broadly classed as adsorption and oxidation. Adsorption of manganese (II) occurs rapidly and is completed in less than five minutes under conditions prevailing in a floe blanket clarifier. The extent of adsorption is determined by pH, iron and manganese concentrations. Manganese adsorption is relatively insensitive to the concentration of other cations and anions present in natural water, although large increases in ionic strength increase the adsorption ratio. As pH and manganese concentrations increase, there appears to be a transition from non specific to specific adsorption of manganese. Indeed specific adsorption would appear to be the precursor of oxidation of adsorbed manganese. Specifically adsorbed manganese is oxidised at the iron (III) oxide surface when solution pH approaches 8.5- The initial reaction product is presumed to be a manganese (III) oxide. The extent to which these removal processes contribute to manganese removal at plant scale will depend on the retention time of both liquid and solid components of flow within the clarifier, as well as clarifier pH. The retention time of solids in the floe blanket is considered to be particularly critical. If adsorbed manganese is discharged from the floe blanket at a higher rate than it is oxidised, then the contribution of oxidation to manganese removal will be insignificant.